Alpha Centauri in Perspective

by Paul Gilster on December 27, 2012

In his new article on Alpha Centauri in Astronomy & Geophysics, Martin Beech (Campion College, University of Regina) noted that the Alpha Centauri stars seem to go through waves of scientific interest. Beech used Google’s Ngram Viewer to look for references to the system in both the scientific literature as well as general magazines and newspapers, finding that there is a 30-year interval between peaks of interest. The figure is suspiciously generational, and Beech wonders whether it reflects an awakening of interest in this nearby system as each generation of scientists and publishers arises.

I mentioned on Christmas Eve that the Beech paper was a real gift for the holidays, and for those of us who try to track developments about Alpha Centauri, it certainly is, drawing together recent work and commenting with care on the findings. The big issue for now is the existence of planets around these stars, a question Centauri B b will begin to answer if it can be confirmed. Everyone from astrophysicists to science fiction authors has noted at one time or another that we may have planets around all three of these stars, no doubt fueling that thirty-year spike Beech identified.

Image: Despite the overexposure of Centauri A and B that fuses the two stars into one at top left, I like it because it reveals Proxima Centauri (shown by the arrow at bottom right), an indication of how distant this nearest star to the Sun is from the two larger stars. Credit: 1-Meter Schmidt Telescope, ESO.

I remember a long-ago sixth grade afternoon when I asked the teacher, after a presentation about astronomy, whether the nearest star had planets like ours around it. After class, she slipped me a book to look at whose title is long lost to memory, but I recall it being stated with confidence that planets were all but impossible around binary stars. Now we know better, for we have planets around binaries elsewhere. As for Alpha Centauri, it was in 1997 that Paul Wiegert and Matt Holman showed that planetary orbits were viable here out to about 4 AU around Centauri A and B.

That would seem to rule out gas giants, which presumably would have formed beyond the snow line at roughly the same 4 AU and beyond, but terrestrial planets in closer orbits are still allowed. Beech runs through the recent scholarship for Centauri A and B, most of which shows the likelihood of planet formation, though in at least one case with a serious restriction:

Guedes et al.(2008) and Thébault et al. (2009) additionally find that planets should have formed around a Cen B. Assuming an initial co-planar distribution of planetesimals, Guedes et al. argue that multiple 1–2 MEarth planets might reside within the region 0.5–1.5 au from a Cen B. Taking into account the details of planetesimal encounters and impact velocities, however, Thébault and co-workers argue that planet formation is only favoured within a restricted zone of 0.5 au about a Cen B. At this stage it will be the discovery of additional planets, with orbits well beyond that of a Cen Bb, that will guide the development of next-generation planetesimal accretion models.

For that matter, can we assume that the current orbital characteristics of Centauri A and B are those of the initial system? In the paper by Philippen Thébault and team referenced above, the authors point out that the odds of Alpha Centauri having gone through a close encounter with a nearby star system that could have affected stellar orbits may be as high as 1 in 2. It’s regrettable, then, that we have no knowledge of the cluster that spawned Alpha Centauri or the effects such encounters might have had. We do know that this system formed from a different cluster than the Sun — our stars are close now but the condition is temporary, and Alpha Centauri was likely 1 to 2 billion years old when our Solar System was beginning to form.

You’ll find most of the papers Beech refers to, the recent ones anyway, discussed in earlier Centauri Dreams posts, and you can use the search function to track them down by author. The same is true of work on Proxima, the M-dwarf that accompanies Centauri A and B, or at least is located at roughly the same distance and shares the same proper motion. It’s hard to believe that the three don’t form a single system, especially given their common chemical composition, but it has always been a tough call going back to the work of Joan Gijsbertus Voûte, who announced the first parallax findings about Proxima in 1917. Voûte pondered whether the three stars were physically connected or ‘members of the same drift,’ and Beech notes that despite the following century of observations, we are still working on the same question.

It’s also been amply demonstrated that both gas giants and rocky worlds can form around M-dwarfs, so while we’re gradually ruling out larger planets around Proxima, we’re far from being able to declare there are no terrestrial planets orbiting it. Here again we’ve run through the numbers in Centauri Dreams in recent memory, but Beech sifts through the findings, which increasingly exclude planets of Jupiter-mass, and he notes in particular the work of Michael Endl and team, which argues that any planet greater than two Earth masses within Proxima’s habitable zone should have been detected by now. That leaves room for smaller worlds in interesting places.

We can also exclude various kinds of planets around Centauri A and B:

With respect to a Cen AB, the five-year radial velocity survey starting in 1992 conducted by Endl et al. (2001) revealed the following constraints: for a Cen A there are no planets more massive than 1 MJupiter within 2 au, and no 2 MJupiter or larger mass planets within 4 au; for a Cen B there are no planets more massive than 1.5 Jupiter within 2 au, and no planets more massive than 2.5 MJupiter within 4 au. On the larger scale, a deep CCD imaging survey of the region immediately surrounding a Cen AB revealed no co-moving companions with masses greater than 15 MJupiter at distances 100–300 au (Kervella and Thévenin 2007).

Thus the search goes on, and Beech believes the current surveys will be able to dampen the noise in the radial velocity signal of an Earth mass planet in a 1 AU orbit around Centauri A or B with about five more years of data gathering. Although the HARPS team at La Silla was first with an Alpha Centauri planet candidate, the hunt for an Earth-class world in a habitable orbit has only intensified. Dedicated Alpha Centauri search programs are in progress not just via HARPS but also through a Yale University program at Cerro Tololo (this one backed by the Planetary Society as well) and through the HERCULES spectrograph at Mt. John Observatory in New Zealand.

“the HERCULES spectrograph at Mt. John Observatory in New Zealand.” is located in “The land of the long white cloud” so is at a distinct disadvantage in observing nights with only 20% of nights photometric (although HERCULES can do spectroscopy on some substandard nights). Having said that, the sky is beautiful here when it isn’t raining!

@Daniel Suggs – yes the Mt John panorama is current. NZ has 16 people per km^2, but half of them live in just 2 cities, there is a lot of open space. Mt John has smaller scopes and a lower elevation than Lick, but the sky is amazingly dark.

When I worked at the Miami Space Transit Planetarium from the late 1980s to the early 1990s, I knew the late Arthur P. Smith, who helped found both the planetarium and the associated Miami Museum of Science (he was also a president of the Southern Cross Astronomical Society). Earlier in his life, he had had some unusual experiences regarding good astronomical “seeing” conditions:

He told me that during his sea-going travels in the Pacific in his youth, he had brought along a small telescope and took astronomical photographs from the equatorial “doldrums” for professional astronomers he knew (he knew Harlow Shapley, among others). They were surprised to see that his photographs were consistently sharper than those taken from high-altitude observatories using comparable magnifications and exposure times. He concluded that the constantly-moving (even if thinner and clear) air moving up the observatories’ mountainsides was more harmful to astronomical photographs than taking such photographs at sea level under clear *and* calm air at the equator.

At Mt. John’s latitude Alpha Centauri is circumpolar, never sets. The planet hunt program there may not have the best equipment but they have a better location in the sense that Alpha Centauri is available for observing year round. Perfect for long term observing like the ongoing planet search.

This reminds me of the attractive advantages provided by the circumpolar sky. There is an increased interest in high latitude observatories. For example the University of Toronto is planning on placing a .5 meter telescope at a place called Eureka (no lie) in the high Arctic at 80 degrees North. For the Southern sky a brass monkey place called Dome C in Antarctica may have the best astronomical seeing that can be obtained from Earth’s surface.
Advantages to be gained from building large observatories in such distant locations are the extremely long observing runs provided by the Polar night. As well the cold dry air improves the astronomical seeing to near space-based telescope values. The extremely dry air allows extended infrared observations. 10 meter or larger telescopes could be built in the high Arctic and Antarctica for a small fraction of the cost of a space-based scopes and provide near space-based capability. They can be built at existing research stations eg. Eureka, Canada and Dome C, Antarctica. Having 2 large telescopes near the poles ( 75 degrees South and 80 degrees North) covers the entire sky. What great possibilities for planet hunting and other astronomical studies! Ofcourse they will have to be idled during the Polar summer but what the heck, you can’t have everything.

I trust I’ve convinced all the Centauri Dreams readers by now, if only the U.S. and Canadian governments could be persuaded to pony up the funding.

And to Joy, that was a lovely panorama of the St. John observatory, thanks.

Mike: “10 meter or larger telescopes could be built in the high Arctic and Antarctica for a small fraction of the cost of a space-based scopes and provide near space-based capability”.

Mike, that is indeed very relevant and I have been wondering about this. Would an earth-based optical telescope ever be able to directly image and spectro-analyze (earthlike) planets for biosignatures, since our atmosphere absorbs so much infrared? Or will space-based telescopes always be needed for this?

Hi Mike,
Another thumb up for the Antarctic domes! However, to get the best resolution the telescope would have to be at the top of a 20 to 40 + meter tower to get above the boundary layer. Even so, it would be worth it for a big ground based IR telescope with a coronagraph, which is what you need for planet imaging.
PS: link to info on the robotic PLATO scope at dome A http://mcba11.phys.unsw.edu.au/~plato/

(Of course they [observatories in the Earth’s polar regions] will have to be idled during the Polar summer but what the heck, you can’t have everything.)

That’s when the “night owl” astronomers would be ‘rotated home,’ to be relieved by the incoming crew of solar (and perhaps also lunar) astronomy observers… If two such dual-mode observatories were established (with one near each pole), year-round solar observations would be possible. For the real “die-hards” among the solar astronomers, they could fly north and south to “follow the Sun” without ever experiencing jet-lag! :-) The British, who have historically had a special interest in solar astronomy (maybe because they seldom see the Sun?), might be good partners in–and strong advocates for–setting up such observatories.

Joy: thanks for the great panorama and for mentioning Dome A, which may actually be a better (if even MORE desolate) location than Dome C (where Concordia station has been open year round for several years now). I heard a talk a few years ago by one of the astronomers involved with PLATO and he called the people who did the overland trips to service it “our heroes”. To get above the boundary layer I always thought that pykrete would be the perfect building material. I mentioned this to one of the Ozzie astronomers involved in site instrumentation in the Antarctic that I happened to share an office with for a few months and he didn’t think much of the idea, darnit!

Points to note: Alpha Centauri A is slightly more favourable for detection of habitable planets by astrometry than Alpha Centauri B, which is more favourable for radial velocity studies.

The conclusion is somewhat sobering though: both RV and astrometric detections would take an enormous amount of observing time. Transits would enable a detection but obviously would require a good deal of luck with the orientation of the planetary orbits.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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